TH400 Transmission Rebuild - Trans-Former

With all the emphasis on g-machines, 17-inch wheels and making vintage cars handle like new ones, sometimes it's easy to lose sight of the street-strip cars that still make up a huge portion-perhaps the majority-of the hobby. They don't have fuel-injected LS engines under the hood or electronically-controlled transmissions. Heck, most don't even have overdrive. Just big tires out big, smaller ones up front and lots of horsepower under the hood.

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In future issues, you'll be reading about a '72 Nova SS we're building that's very similar to the vehicle described in the preceding paragraph. It's mini-tubbed, set up for a big-block and we procured it minus the engine and trans. Since it had been home to a Rat for many years, we figured it should get another. To that end, we're making use of the Dart 509 you read about a few issues back (Real Fine 509, November and December 2009 issue). To review, Dart took its Big M block and punched the cylinder bores to 4.500-inches each. Then it took a standard Eagle 4.00-inch stroke crankshaft and came up with 509ci. The top-end consists of a Comp Cams hydraulic roller bumpstick, Dart Pro 1 310cc heads, and Dart single-plane intake manifold. All told, this baby cranked out a stout 668 hp and 623 lb-ft-no doubt, this puppy will be a nasty little street machine once finished.

The Dart 509 power curve dictates the need for a strong unit in the trans tunnel. For that, we skipped over the popular TH350 in favor of the stronger TH400. The GM factory officially refers to a 400 trans as a Turbo-Hydramatic (TH) and it first appeared in the GM lineup back in '64. The 400 moniker's origins are unknown. The transmission had a long and successful life in a variety of GM vehicles until it was phased out sometime in the '80s, giving way to the four-speed overdrive transmissions. In the world of muscle cars, this transmission was found behind many a muscle car, including big-block Chevelles and Corvettes. A lot of racers still use the Powerglide two-speed, but we opted for the TH400 over a 'Glide because of its three-speed classification-making it fun on the street. The three forward gears consist of a 2.48 first gear, 1.48 second gear, and a final 1.00 ratio for third.

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We didn't just dig a TH400 out of the junkyard and throw it in this X-body; our plan pointed us to Gaithersburg, Maryland, which serves as home to Performance Automatic (PA). The folks at PA took our junkyard TH400, cleaned it up, and turned it into its Street Smart Stage II transmission. The Street Smart Stage II is rated to 650 rear wheel hp, which is plenty for our big-block powerplant, even if we decide to hit it with a shot down the road. Our engine cranked out 668 hp at the crank, which differs from rear-wheel hp by roughly 15-20 percent loss through the drivetrain. PA also offers a few other versions of the TH400 with varying hp ratings and different options to fit most budgets.

Our junkyard slush box was not in stellar shape and PA disassembled and inspected all of the components before rebuilding it. Perhaps the only bump in the road that PA's Gerald Nusser discovered was a broken linkage. "This is rare but it is something we fix by just welding it. This case was fixable, but in other scenarios, we will discard the case rather than attempt to fix it," he stated. PA keeps a vast inventory of TH400 transmissions (as well as other trans styles) in the event a customer has an irreparable case or simply doesn't have a core to trade in.

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The Stage II trans features better shift calibrations, heavy-duty Alto Red clutches and bands, and upgraded sprags. In addition to those mods, PA also added a new transmission pan and drain plug. The biggest highlight of the Stage II packages is the forward and direct drum each carrying five Alto Red high-performance clutches. The stock drums contain only four clutches.

Our junkyard slush box is now a solid transmission that is ready, willing, and able to hold up behind the Dart 509ci. Tune in to future issues as we continue to grind away on this unsuspecting Nova.

MORE PHOTOS

Here is our TH400, which was procured from a junkyard. Its guts probably haven’t seen the light of day since it was put together over 20 years ago.

The Performance Automatic crew disassembled the transmission. The fluid was rather ripe and made rear-end lube seem like roses. The wearable items like clutches and bands were discarded, as were some other items that PA replaces. The parts that were kept were cleaned in an industrial hot-tank.

The shifter cable lever was broken in this case and according Nusser this was easily repairable. If there were other problems with the case, it would have been discarded. A local welding shop repaired the lever in about a half an hour.

The stock clutches were fried, as you can see scorch marks on the surface. PA replaces the four clutches with five Alto Red high-performance clutches for better holding power.

In the foreground is a new direct drum that PA utilizes in its Stage II builds. The drum uses a 34-element sprag. In the background is the stock drum with only an eight element sprag. According to Nusser, this drum is hooked up to second gear and helps beef that gear up as well.

The return springs for the forward drum were cleaned in the hot tank and reinstalled.

Nusser installs five Alto Red clutches to each drum.

The manual low/reverse band doesn’t see a lot of load, therefore it doesn’t get worn out. PA inspects each band on a case-by-case basis and determine if the stock one can be reused. Despite the poor shape our junkyard trans was in when it got there, Nusser decided to reuse the stock manual low/reverse band. PA does replace the kick-down band with an Alto high-performance one.

The direct drum/third gear gets installed into the transmission case. The guts of the transmission go back into the case rather quickly. The transmission build-up was accomplished in just one afternoon, after the initial disassembly, inspection, cleaning, and upgrades.

The front pump is oiled and resealed before being installed back in the case.

The pressure regulator spring, which increases line pressure for firmer shifts, is located inside the front pump and is replaced. It is included in a shift-kit.

A new separator plate and valvebody gasket is installed. The separator plate features larger holes, which provides firmer and more positive shifts.

Here is the kick-down solenoid, which requires a power supply to kick down the transmission. Some people leave the power off this solenoid and manually shift the transmission.

The stock valvebody is bolted back on. The firmer shifts are accomplished through the addition of a PA shift kit and the replacement spring in the front pump.

A new trans filter is bolted on top of the valvebody, before the new PA pan is bolted on.

Here is the governor being reinstalled, which regulates the automatic shifting.

Nusser reinstalls the modulator valve, which runs off engine vacuum. When under no vacuum and high load (WOT), it delays the shift to a higher rpm.

This is vacuum hook-up for the modulator valve.

Once the transmission is completed, Nusser sprays the outside surface black. The transmission is then transferred PA’s in-house trans dyno test stand. Every transmission is tested on the stand to ensure it shifts properly and doesn’t have any leaks. Once it checks out, the transmission is moved over to the shipping department and sent on its way to the new owner.